The present invention relates to coating applicators in general and to apparatus for applying coatings to moving substrates in particular.
Paper of specialized performance characteristics may be created by applying a thin layer of coating material to one or both sides of the paper. One type of coating fluid is a mixture of a fine plate-like mineral, typically clay or particulate calcium carbonate; coloring agents, typically titanium dioxide for a white sheet; and a binder which may be of the organic type or of a synthetic composition. Another type of fluid is a starch and water solution used in sizing applications. Coated paper is typically used in magazines, commercial catalogs and advertising inserts in newspapers. The coated paper may be formed with a smooth bright surface which improves the readability of the text and the quality of photographic reproductions. Coated papers are divided into a number of grades. The higher value grades, the so-called coated free-sheet, are formed of paper fibers wherein the lignin has been removed by digestion. Less expensive grades of coated paper contain ten percent or more ground-wood pulp which is less expensive than pulp formed by digestion.
Coated papers are often used for high-quality printing or in other applications where visible variations in coating weight would significantly detract from the appearance of the paper. It is therefore of key concern to maintain coating thickness consistency across the width of the treated web. Greater efficiency and cost control in papermaking has driven the construction of ever wider papermaking machines, sometimes of 300-400 inches or more. In conventional fountain applicators, a single supply chamber extends the full width of the web in the cross machine direction. This supply chamber may be fed from one or both ends. To minimize fall off of coating ejected from a nozzle which terminates the supply chamber, coating is supplied at a high pressure. Nevertheless, such coaters are prone to heavier coating application at the ends.
Furthermore, the heated coatings which are frequently employed can, over the extended cross machine width of the coater head, result in temperature gradients which cause bowing of the head with resultant coat weight variations.
What is needed is a papermaking fountain applicator which may be operated at lower pressures while still supplying consistent coating levels to the substrate in the cross machine direction.